1. Field of the Invention
The present invention relates to a brushless motor which is installed as a drive member of a compact disc player, a tape player or the like.
2. Description of the Prior Art
FIG. 4 is a sectional view of a brushless motor in the prior art.
The rotary shaft 1 is rotatably supported by a thrust bearing 2 and a radial bearing 3. A rotor yoke 4 is fixed to the rotary shaft 1, and a ring-shaped rotor magnet 5 is fixed to a lower surface of the rotor yoke 4. A coil substrate 7 is fixed on the stationary plate 6. The coil substrate 7 is flexible. A plurality of stator coils 8 are fixed on an upper surface of the coil substrate 7. The stator coils 8 are wound in nearly square form and arranged at regular intervals as shown in FIG. 5. A lead plate 7a extends integrally from the edge portion of the coil substrate 7. A lead pattern (not shown) is formed on the lead plate 7a, and current is supplied to each stator coil 8 through the lead pattern. A detecting substrate 9 is fixed on the stator coils 8. The detecting substrate 9 is also a flexible substrate. The detecting substrate 9 is provided with a pattern 10 for detecting the rotational speed as shown in FIG. 6. A lead plate 9a is installed integrally to the side edge portion of the detecting substrate 9, and lead patterns 10a are formed on the lead plate 9a. A signal from the pattern 10 for detecting the rotational speed is taken out by the lead patterns 10a.
As shown in FIG. 7, the rotor magnet 5 comprises a main magnetizing member 5a formed on an outer circumferential portion, and an FG magnetizing member 5b for detecting the rotational speed formed on an inner diameter side. In the main magnetizing member 5a, N-poles and S-poles are alternately arranged in the circumferential direction and magnetized. The stator coil 8 is constituted so that current flows in the radial direction at linear parts 8a and 8b. Electromotive force in the rotational direction acts on the rotor magnet 5 according to the current flowing through the linear parts 8a and 8b and the polarity of the main magnetizing member 5a opposed to the current.
In the FG magnetizing member 5b of the rotor magnet 5, N-poles and S-poles are alternately formed in the circumferential direction at shorter pitch than that of the main magnetizing member 5a. On the other hand, the detecting pattern 10 as shown in FIG. 6 comprises detecting parts 10b arranged at short pitch so that current flows in the radial direction. The pattern 10 for detecting the rotational speed is opposed to the FG magnetizing member 5b as shown in FIG. 8. If the rotor magnet 5 is rotated, pulses are outputted from the pattern 10 for detecting the rotational speed corresponding to the magnetic poles of the FG magnetizing member 5b. The rotational speed of the rotor magnet 5 is detected according to the pulse output.
In the brushless motor in the prior art, the detecting substrate 9 is formed as shown in FIG. 6, and the lead patterns 10a connected to the pattern 10 for detecting the rotational speed extend in parallel to each other at a small spacing. Furthermore, the lead patterns 10a cross the main magnetizing member 5a of the rotor magnet 5 and extend outward as shown in FIG. 8. Consequently, if the rotor magnet 5 is driven to rotate, the main magnetizing member 5a is moved across the lead patterns 10a so that electromotive force is produced in the lead patterns 10a by means of the magnetic force of the main magnetizing member 5a. When the two lead patterns 10a are disposed across magnetic poles of the same polarity, e.g., N-poles of the main magnetizing member 5a, the electromotive force in the same direction acts on the two lead patterns 10a (refer to .alpha. in FIG. 8). In this case, the electromotive force in the two lead patterns 10a is canceled, and current does not flow in the lead patterns 10a. On the contrary, as shown in FIG. 8, when the two lead patterns 10a ride on the border of the N-pole and the S-pole, the electromotive force in different directions is generated in the two lead patterns 10a as shown in .beta. of FIG. 8. Since the lead patterns 10a are connected to both ends of the pattern 10 for detecting the rotational speed in series with each other, unwanted current flows through the pattern 10 for detecting the rotational speed and the lead patterns 10a by means of the electromotive force shown at .beta.. If the unwanted current is produced, as shown in FIG. 9, regular distortion is produced in the rotational speed detecting pulses P outputted from the lead patterns 10a.